BS3: Add optional theme (fixes #8706)

## Preview

![theme template for bootstrap](https://f.cloud.github.com/assets/98681/981942/91778c10-07aa-11e3-9234-113e06e0bb1a.png)

## What's changed

Just finished the first pass at our optional theme to make Bootstrap 3 look a bit more like Bootstrap 2. It includes a first pass at adding visual effects for nearly all our components.

Specifically:

* Added a new `theme.less` file to house all our theme-specific styles (which also imports vars and mixins forr use)
* Compiles the theme file into two CSS files, `bootstrap-theme.css` and `bootstrap-theme.min.css`
* Adds new example, "Bootstrap theme", to the Getting Started examples section in the docs

I'm sure I missed something or need to tweak something else, but we can address that in time. In the future, we'll do our best to make it so folks can simply change a variable and enable the theme rather than including a secondary file. For now though, this will suffice for v3.